1. Brief Description of the Invention
The present invention relates to improved thickening compositions, often referred to as rheological additives, used to provide viscosity control, flow, leveling and other Theological properties to aqueous systems such as paints and coatings, inks, drilling fluids, adhesives, construction materials and household products. The invention also includes a method of making such improved thickening compositions, aqueous systems containing such thickening compositions and a method of thickening such aqueous systems.
The invention hereof involves the synthesis of a new family of rheological additives by combining, through a reaction process, chemicals used to make polyurethane polymer compositions with chemicals used to make alkali-swellable copolymer compositions into the same copolymer molecule to provide good flow and leveling and gloss retention that polyurethane associative thickeners impart to water-based systems, together with the good sag resistance that alkali swellable copolymers impart to such water-based systems.
2. Description of the Prior Art
In General
It has long been known that rheological additives, which are added at only a small weight percent to an aqueous system, are needed in order to modify the rheology of aqueous systems such as paints, decorative and protective coatings, paper coatings, household detergents, cosmetics and personal care items, adhesives and sealants, inks, drilling fluids, and the like to improve their application properties.
Thixotropes are rheological additives which impart a three dimensional structure to liquid systems as expressed by high viscosity at low shear rates. When the system is sheared at high shear rates, this structure is broken down, resulting in a decrease in viscosity. The structure recovers when the external force is removed. The rate of recovery determines the application properties of the system such as sag resistance and leveling. Rheological additives are added at about 0.01% to about 10% based on the total weight of the system to be thickened. Often the terms thixotrope and Theological additive are used interchangeably.
Many Theological additives for water based systems are available: natural, modified--natural and synthetic. Natural rheological additives include guar gum, pectin, casein, carrageanan, xanthan gum and alginates. Modified additives include--modified celluloses, most particularly methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and carboxymethyl cellulose.
Modified-natural thickeners, such as hydroxyethyl cellulose, have dominated portions of the latex paint thickener market almost since the introduction of latex paint on a commercial scale. Hydroxyethyl cellulose suffers from several disadvantages, not the least of which is its relatively high cost. In some formulations, hydroxyethyl cellulose is responsible for poor spatter resistance and poor leveling. Hydroxyethyl cellulose thickeners as partially natural products are also particularly subject to bacterial and enzymatic-induced degradation, resulting in loss of viscosity and other flow properties, and therefore require the incorporation of anti-biodegradation preservatives when used in aqueous systems. In addition, these types of thickeners swell rapidly in water to form lumps which are not readily dispersed. The proper addition of hydroxyethyl cellulose thickeners, therefore, often requires slow metered addition, resulting in much longer mixing and dilution times than desired.
For these reasons, a search has long continued for other types of thickeners to replace hydroxyethyl cellulose and similar cellulosic thickeners which (1) can be employed as a post-additive directly to aqueous compositions, particularly latex paint and ink compositions, (2) are not biodegradable, (3) yield a lower overall manufacturing cost for equivalent performance with respect to the Theological properties of latex compositions into which the thickeners are mixed, (4) provide better viscosity--control properties, and (5) improve storage stability and pigment suspension.
In the last twenty years, synthetic rheological additives have assumed increased commercial importance. These newer synthetic thickeners provide improved pigment suspension and improved application properties. In water based systems including latex paints, it is highly desired that such systems be flowable during their application, but that they not sag after they have been applied. It is further desired that the coating will form a homogeneous film and a smooth flat surface. Such rheological additives may be added to the aqueous latex system at any time including during the grinding stage or after the pigment has been dispersed, i.e., as a post-additive.
Various synthetic polymeric thickeners have been prepared, for example, polyether-polyurethane associative rheological additives and alkali swellable thickeners.
Two patents issued to Rheox Inc., the assignee herein, describe a family of polyether polyurethane thickening compositions. These patents, U.S. Pat. Nos. 4,499,233 and 5,023,309, describe these rheological additives as the reaction product of polyisocyanates, polyether polyols, modifying agents which contain at least one pendant internal hydrophobic group and at least two active hydrogen moieties terminated by a chemical capping agent.
Another type of polyurethane synthetic thickener used extensively in commercial applications is described in U.S. Pat. Nos. 4,079,028 and 4,155,892. The thickener is described as prepared by reacting at least one water soluble polyether polyol with at least one monofunctional hydrophobic organic compound, usually an organic monoisocyanate. A related type of synthetic polyurethane thickener is a water-soluble thermoplastic organic polymer having a number of monovalent hydrophobic groups incorporated in the internal portion of the polymer molecule. U.S. Pat. Nos. 4,496,708 and 4,426,485, issued to Union Carbide Corporation, describe polyurethane thickeners which are water-soluble comb polymers containing a number of pendant internal hydrophobic groups.
The above-described rheological additives, have been often generally referred to by the term polyurethane associative thickeners. Associative thickeners are so-called because the mechanism by which they thicken involves hydrophobic associations between the hydrophobic species in the thickener molecules and other hydrophobic surfaces, either on other thickener molecules or on latex particles, pigments or micelles in the system to be thickened.
Alkali swellable thickeners are manufactured by copolymerizing two or more monomers, one or more of which is carboxyl-containing. These polymers may have an essentially linear structure, a branched structure or a three dimensional network structure caused by cross-linking. One of the first such polymers is described in Rheox U.S. Pat. No. 4,226,754, issued in 1980, which in a preferred embodiment discloses a polymer made by the reaction of (a) an ester of methacrylic acid, (b) methacrylic acid and (c) a vinyl ester of a saturated aliphatic carboxylic acid. This type of thickener has often been referred to as an alkali-swellable or alkali soluble latex copolymeric thickener as it contains carboxylic acid groups in sufficient quantity to render the polymer soluble in water following neutralization with a water-soluble base.
The composition of alkali swellable thickeners has been extended by the introduction of hydrophobic entities into the polymeric backbone. U.S. Pat. No. 4,384,096 describes an improved water soluble thickener which discloses a polymer which is the reaction product of a C.sub.3 -C.sub.8 .alpha., .beta.-ethylenically unsaturated carboxylic acid monomer, a nonionic vinyl monomer, and from 1% to 30% of a defined vinyl surfactant ester. One of the two vinyl surfactant esters shown in the patent is an alkylphenoxypoly (ethyleneoxy) ethyl acrylate, terminated on one end with a C.sub.8 -C.sub.16 alkyl phenyl group. U.S. Pat. No. 4,138,381 shows a reaction product of an unsaturated carboxylic acid of 3 to 6 carbon atoms, alkyl acrylate or alkyl methacrylate, and an ester containing an alkyl phenyl group, where the alkyl group has from 8 to 20 carbon atoms.
European Pat. No. 13,836 describes an acrylic emulsion copolymer useful as a thickener, made by the emulsion copolymerization of four types of monomers, which would include a) acrylic or methacrylic acid, b) alkyl acrylate or methacrylate, c) an ethoxylated ester of acrylic or methacrylic acid having a hydrophobic group and d) an optional polyethylenically unsaturated monomer. A 1987 article in Volume 57 of the Proceedings of the ACS Division of Polymeric Materials on pages 476 to 481 describes adding by solvation an alkyl polyethylenoxy ether surfactant to a polymer with a polyacrylamide backbone.
A similar variant of a polymeric thickener for aqueous paint and drilling muds is described in U.S. Pat. No. 4,421,902. This patent discloses a copolymer which can be the reaction product of monomers including methacrylic acid, ethyl acrylate, optionally a defined copolymerizable ethylenically unsaturated monomer, and a small weight percent of a polyethylenically unsaturated monomer. This patent further describes how a wide range of surfactants can provide enhancement of thickening when added to an aqueous system containing the copolymer of the invention, when the emulsion copolymer is neutralized.
More recently, U.S. Pat. No. 4,668,410 shows a water-soluble copolymer for systems free of mineral oil, used to adjust the viscosity of the system. This polymer is the reaction product of six components, including methacrylic acid, methacrylic acid esters or acrylic acid esters of certain alcohols, vinyl esters and a surface-active unsaturated ester. The surface-active ester is terminated at one end with an aliphatic (C.sub.2 -C.sub.30) -radical, which can be linear or branched, a mono-, di- or tri-alkyl phenyl radical with alkyl groups of 4 to 12 carbon atoms, or a block-copolymeric radical of a specific type. On partial or complete neutralization, the polymer becomes water-soluble or colloidally dispersible in water, and can be used as a thickener.
Urethane-Acrylic Chemistry
Urethane-acrylics are known in the paint and adhesive industries. They have been mostly used as resins or binders. A major application as resins have been low viscosity resins which are cured by chemical reaction or by ultra violet (UV) light to form a crosslinked copolymer. Such compositions are disclosed, for example in European Pat. No. 0511827 and in the Journal of Applied Polymer Science 46 (8) 1339 (1992). Other related compositions are vinyl ester urethane resins (U.S. Pat. No. 3,876,726) and vinyl urethane resins (U.S. Pat. No. 3,929,929). These patents both employ a hydroxy terminated ester of acrylic or methacrylic acid for the purpose of synthesizing a low viscosity resin. U.S. Pat. No. 4,659,780 and U.S. Pat. No. 5,314,942 describe acrylourethane compositions prepared from an acrylic prepolymer, hydroxyethyl acrylate (HEA), hydroxyethyl methacrylate (HEMA), hydroxypropyl acrylate (HPA) or hydroxypropyl methacrylate (BPMA), acrylic or styrenic monomers, a diol, a diisocyanate and a monofunctional alcohol. The latter patent describes a composition which contains reactive groups, reacted with a crosslinking resin such as melamine formaldehyde to form a coating film. U.S. Pat. No. 5,475,038 describes a print laminating adhesive composition comprising an acrylate terminated oligomer of a defined formula derived from the residue of a diisocyanate. U.S. Pat. No. 5,565,508 shows a synthetic resin dispersion prepared using a blocked polyisocyanate and a ethylenically unsaturated monomer. See also recent U.S. Pat. Nos. 5,525,693, 5,480,943 and 5,475,073.
The use of a monoisocyanate with ethylenic unsaturation, such as isocyanatoethyl methacrylate to form a surfactant monomer in an alkali swellable polymer is disclosed in U.S. Pat. No. 5,066,710. A similar approach is disclosed in U.S. Pat. Nos. 4,514,552, 4,722,962, and 4,801,671, describing an alkali soluble thickener prepared from ethylenically unsaturated carboxylic acid, typically methacrylic acid, a monoethylenically unsaturated monomer, typically ethyl acrylate, a nonionic urethane monomer which is the reaction product of a monohydric nonionic surfactant with a monoethylenically unsaturated monoisocyanate and a polyethylenically unsaturated monomer. The last two patents are apparently the basis of the commercial product line known as "Polyphobes" sold by Union Carbide Corporation. The properties of these Polyphobe thickeners are summarized in The American Paint & Coatings Journal, Jul. 3, 1995, p. 51. U.S. Pat. No. 4,730,021 describes a process for preparing aqueous dispersions of urethane acrylate graft copolymers prepared by first reacting an active hydrogen containing vinyl compound with a diisocyanate to produce a monoacrylated diisocyanate adduct useful as binders for coatings and printing inks.
U.S. Pat. No. 4,659,780 shows an ungelled reaction product of an acrylic prepolymer, a diol and a diisocyanate reacted in the presence of a monoalcohol said to be useful in high solids coating compositions. Recent U.S. Pat. No. 5,484,864 describes adhesive compositions with improved bonding formed as the reaction product of a urethane methacrylate and other reactants. Even more recent U.S. Pat. No. 5,594,065 shows a two component coating system based upon crosslinkable hydroxy-containing polymers and a polyisocyanate cross linker.
These products, while effective as the binder in many systems, have not satisfied the need for effective water based thickeners. For these reasons, a search for a polymeric acrylic-urethane thickener has continued. The present invention was developed in response to this search, and the long-felt need it represents for a product which will impart the required sag resistance, viscosity efficiency and good leveling to aqueous systems.
Object of the Invention It is a specific object of the claimed invention to provide a highly effective urethane-acrylic rheological additive for aqueous compositions, particularly aqueous paints and coatings. Such compositions specifically include latex systems, paints, coatings, inks, adhesives, construction materials, household and personal care systems and drilling fluids.
It is a further object of the invention to provide an additive which can easily be made using available chemical materials and current process technology, including known suspension, emulsion and solution polymerization techniques, on a continuous, semi-continuous, or batch basis.
It is a further object of the invention to provide an additive that can be readily dispersed into the system to be thickened, which is easily handled, and which can readily be incorporated.
Other objects, as well as advantages, of the invention will become clear from the following summary and description, or will become apparent to the skilled artisan, as obvious variations of the instant invention.